chore(package): update webpack to version 4.4.1

[rpl]

This pull request supersedes #1927, which fails on travis because webpack 4 also requires webpack-cli to be explicitly installed as a separate dev dependency.

This pull request also set the new webpack mode config property to 'none', which seems to restore most of the default behaviors of webpack3, otherwise the build step raise a number of ModuleConcatenation bailout: Module is not an ECMAScript module warning messages (and an additional warning message which complains about the missing mode config property and warns that webpack 4 will defaults to the new mode: 'production' behavior when the explicit mode is missing).

I also briefly tested locally that the new webpack4 bundle is able to run successfully from the addons-linter bin script (and that the npm run extract-locales script is able to extract the i18n strings as expected, given that it is also based on a webpack config and it could have been affected by a webpack upgrade).

[EnTeQuAk]

I'm still working on investigating using alternative modes but for the time being and to get into contact with webpack 4 mode: none works fine for now so let's get the money!

[EnTeQuAk]

Webpack just released #1948 version 4.5 but let's land this here first and upgrade to anything even more recent later.